The present invention relates to the distance measurement arts. It particularly relates to distance measurement using an ultrasound transmitter and receiver, and will be described with particular reference thereto. However, it is to be appreciated that the invention can be used with any other pulsed waves including electromagnetic waves.
Automotive technology frequently calls for measuring the distance between two objects in a simple and cost-effective manner. Exemplary automotive technology applications include measurement of the distance of a wheel suspension, e.g. the mobile part of a shock absorber, relative to the vehicle body. Such a measurement is used in adaptive running gear adjustment.
Another exemplary application is in the area of airbag control. Typically, the activation of the airbag triggering mechanism and/or the activation of various airbag triggering stages depends upon the position of the affected passenger. In such applications, the position of the seat is typically recorded to provide a rough measure of the distance between the person and the airbag.
Various distance measuring devices are known for these types of applications. In many cases including the aforementioned exemplary applications, there is a need to measure the absolute distance. In these cases, more costly absolute value measurement devices are employed rather than incremental measurement devices. These applications are also usually incompatible with the known method of using an incremental device and determining the absolute distance therefrom by initializing to a known position and then measuring the movement from that position to determine the absolute distance. When recording, for example, the position of a vehicle seat, it would be necessary to move the seat into a known initial position in order to implement absolute distance measurement using an incremental device in the above-described manner. For safety reasons, such initializing would preferably be repeated whenever the triggering mechanism is disconnected from electrical power. Without such re-initializing, inaccurate absolute value measurements are possible, which would result in defective airbag control.
Conventional absolute distance measurement devices for measuring the position of a movable object operate on an inductive or a capacitive basis. During movement of the object to be measured, the inductance or capacitance of a corresponding sensor changes.
In another known absolute distance measurement device type, the position of a person in a motor vehicle seat is measured by means of ultrasound waves. In a typical case, such measurement involves measurement of the distance from the head or upper torso of the person to the airbag.
Another application of ultrasound absolute distance measuring devices is in parking aid devices.
When recording the position of an object by means of ultrasound, the usual method is to measure the transit time of an impulse train from transmission to detection. The detection can be either of the direct ultrasound signal or of its reflection off the target object.
The currently known devices for measuring distances are, however, primarily employable only in specific applications, specifically the movement of an object in a motor vehicle. This applies, in particular, in the case of large regulating distances of the object.
Devices for measuring distance which function on an ultra-sound basis also have the drawback that other objects can interfere with the signal path and thereby lead to faulty control of the corresponding actuators.
Starting from said state of the art, it is an object of the invention to create a device having a simple and cost-effective design, which can be integrated in simple fashion into a vehicle and which permits registration of the position or distance of a preferably movable object from a given point of reference with sufficient safety and accuracy.
The invention includes a telescopic tube having at least two engaging tube elements. A transmitter element is provided on one tube element and at least one receiver element is provided on the other tube element. By using the telescope tube, objects are prevented from unintentionally interfering in the operational space between the transmitter and the receiver. In addition, the construction of the device enables complete pre-assembly of the device before installation on a vehicle. The installation on the vehicle is then quick and easy. The inner tube element of the telescope tube may be replaced as desired by an element which is displaceable in the outer tube element.
According to a preferred embodiment of the invention, the at least one of a transmitter element and a receiver element are provided at a front side or at partition walls of the tube elements and extend vertically relative to the axis of the telescope tube. As a result, the transmitter element and the receiver element can essentially be positioned along the axis of the tube, so that the signal essentially also propagates along the axis of the tube. Consequently, no complicated signal evaluation is required to compensate for the reflections of the signal along the tube wall.
The signal of the transmitter element can also be passed to the telescope tube via rigid or flexible waveguides, e.g. via flexible plastic hose. In this case, it will be appreciated that the length of the waveguide between the transmitter element and the telescope tube or the end of the distance to be measured is preferably taken into consideration in determining the distance from the signal transit time and the signal velocity.
In like fashion, it is possible to transmit the receiver signal also to the transmitter element via a corresponding waveguide, whereby said length must also be taken into consideration in determining the distance. This applies not only with respect to the length of the waveguide, but also to a partial length of the telescope tube, if the transmitter or receiver elements are not arranged at the end points of the distance to be measured. In this instance, the corresponding portion of the telescope tube functions as part of the aforementioned waveguide.
Furthermore, in the preferred embodiment of the invention, a second receiver element is provided on the tube upon which the transmitter element is arranged. The second receiver is spaced apart from the transmitter by a known distance. Said second receiver element serves for determining the signal velocity by measuring the signal transit time between the transmitter element and the second receiver element, making use of the known distance between these two elements. The determined signal velocity is then used for determining a highly accurate absolute value for the distance between the transmitter element and the first receiver element.
The second receiver element is also selectively disposed at a coupling waveguide in operative communication with the telescope tube. Alternatively, the second receiver element can be connected by means of an additional decoupling waveguide in operative communication with the coupling waveguide or with the telescope.
It is possible to compensate in this fashion for variations in the signal frequency or for temperature variations that affect the signal velocity through temperature variations in the density of the medium.
It is to be appreciated that the second receiver element is selectively provided at the tube element at which the first receiver element is arranged. Alternatively, the second receiver element is arranged at or in a decoupling waveguide between the telescope tube and the receiver element. With this arrangement, the signal velocity is determined using knowledge of the distance between the two receiver elements and the recorded signal transit time between the two receiver elements.
The invention is preferably employed to measure distance using electromagnetic waves, including waves within the optical spectrum, or using acoustic waves. Measurement by the invention using acoustic waves is preferred because the acoustic transmitter and receiver elements are cost-effective. The required evaluation electronics can also be of simple and cost-effective construction due to the relatively low signal velocity (i.e., the diffusion velocity of the signal) between the transmitter and the receiver.
According to another embodiment, the peripheral walls of the telescope tube are provided with perforations for attenuating signal portions that reflect off the wall. These perforations reduce the reflectivity of the walls. Optionally, appropriate damping materials are provided at the interior walls of the tube elements. Optionally, the interior walls are fitted with appropriate geometric attenuation elements.
The receiver element is preferably provided at the end of the telescope tube opposite the transmitter element, i.e. in an appropriate end region of the telescope tube. The receiver element is alternatively be at the same end of the telescope tube as the transmitter element. In the former case, the transmitter signal is registered directly by the receiver element. In the latter case the transmitter signal is registered at a reflecting wall provided at the appropriate end inside at least the one additional tube element.
In yet another embodiment, the transmitter element and the receiver element are formed as an integrated transmitter/receiver element. In the case of an ultra-sound converter, the converter serves as both the transmitter and the receiving element. With this type of integrated design, however, it is to be appreciated that the reactance time of the converter must be lower than the lowest signal transit time to be measured. The reactance time is the time that the converter maintains oscillations after being switched off. During the reactance time period, the converter cannot function with adequate precision as a receiving element.
According to yet another embodiment of the invention, an evaluation-and-control unit, which acts upon the transmitter element and to which signals are passed for evaluation from the receiver elements, is integrated with the telescoping tube.